The growth of mammalian cells in vitro usually requires the addition of a chemically defined nutrient medium supplemented with serum. The introduction of serum to the cell culture system as an undefined biological component, however, contributes to the variability of the in vitro biochemical results. Furthermore, the use of serum in a cell growth medium contributes to the growth of unwanted cells: that is, it is difficult to propagate a specific cell culture when serum is included in the growth medium. Furthermore, serum is expensive and in some instances is toxic to certain cells.
The possibility of growing cells in serum-free media has been suggested in an article entitled "The Growth of Cells in Serum-Free Hormone-Supplemented Media" by Bottenstein et al., Methods in Enzymology, Vol. LVIII, p. 94 et seq. The cells reported to be grown by the process disclosed in that article, however, were abnormal cells. Indeed, prior to the present invention, no one has been able to grow normal mammalian cells in the complete absence of serum. An additional article setting forth a process for growing mammalian cells in a serum-free system is "Hormonal Requirements of Baby Hamster Kidney Cells in Culture" by Maciag et al., Cell Biology International Report, January, 1980. The cell culture grown by the procedure set forth in the foregoing article, however, was a BHK-21 cell culture obtained from the American Type Culture Collection which is an abnormal cell.
In connection with the foregoing, the term "abnormal" mammalian cell is intended to describe a cell which is usually derived from a tumor and which has an infinite life span. On the other hand, the term "normal" mammalian cell as used throughout this specification and claims describes a cell which is non-tumor derived, and which has a finite life span in tissue culture.